1. Field of the Invention
This invention relates generally to quadrature modulators, and, more specifically, to reducing or eliminating the unwanted sideband in a signal derived from the output of a quadrature modulator.
2. Background
In a quadrature modulator, identified with numeral 30 in FIG. 1A, a complex baseband signal, i.e., a baseband signal having I and Q components, BBI and BBQ, assumed to be in quadrature (out of phase by 90xc2x0), is mixed with a complex local oscillator signal, also having I and Q components, LOI and LOQ, and also assumed to be in quadrature, to form an output signal. The output signal may be at the desired RF transmit frequency in the case of a direct conversion transceiver, or it may be at an intermediate frequency which must in subsequent stages be unconverted to the desired RF transmit frequency.
The I component of the baseband signal, BBI, is mixed, through mixer 31, with the I component of the local oscillator signal, LOI, and the Q component of the baseband signal, BBQ, is mixed, through mixer 32, with the Q component of the local oscillator signal, LOQ. The outputs of mixers 31 and 32 are then combined, through combiner 33, to form the output signal of the quadrature modulator.
If the I and Q components of the baseband signal, and the I and Q components of the local oscillator signal, are perfectly in quadrature, i.e., out of phase by 90xc2x0 exactly, and the components of the quadrature modulator are perfectly accurate, only the xe2x80x9cwantedxe2x80x9d sideband will appear at the output of the quadrature modulator, and no energy will appear in the xe2x80x9cunwantedxe2x80x9d sideband. This follows mathematically as follows: assuming BBI can be represented as A cos(xcfx89BBt+xcfx80/2), BBQ as A cos(xcfx89BBt), LOI as B cos(xcfx89LOt+xcfx80/2) , and LOQ as B cos(xcfx89LOt) , then the output of the quadrature modulator, (BBIxc3x97LOI)+(BBQxc3x97LOQ), reduces, through known mathematical identities, to Axc3x97B cos([xcfx89BB+xcfx89LO]t). This is the xe2x80x9cwantedxe2x80x9d sideband. As can be seen, there is no component at the xe2x80x9cunwantedxe2x80x9d sideband, i.e., at the frequency xcfx89LOxe2x88x92xcfx89BB.
However, in the real word, there will be some inaccuracy in the quadrature of the baseband or local oscillator signals or in the components of the quadrature modulator. That will result in some energy at the unwanted sideband frequency. This situation is illustrated in FIG. 2B, which is a frequency domain representation of the output of the quadrature modulator in the case in which there is some inaccuracy in the quadrature of the baseband or local oscillator signals or in the components of the quadrature modulator. Representing baseband as a single frequency, the wanted sideband is identified with numeral 50, while the unwanted sideband is identified with numeral 51. (Representing baseband as a range of frequencies, the wanted sideband is identified with numeral 52, while the unwanted sideband is identified with numeral 53).
Typically, as illustrated, the amplitude of the unwanted sideband is less than that of the wanted sideband. However, in extreme cases, when the quadrature inaccuracy is large, the amplitude of the unwanted sideband can approach that of the wanted sideband.
In the case in which the transmitted signal is a phase-modulated signal, the presence of the unwanted sideband in the output of the quadrature modulator translates into phase error in the transmitted signal. However, current GSM standards impose tight limits on the phase error of the transmitted signal.
In xe2x80x9cAn ISM band Transceiver Chip for Digital Spread Spectrum Communicationxe2x80x9d, ESSCIRC 97, a circuit for generating the LOI and LOQ inputs is described. The circuit is illustrated in FIG. 1B. The LOI and LOQ signals are provided by a divide by two circuit comprising two D-type flip-flops 2 and 3 driven by VCO 1. These signals are input to phase detector 4, which outputs a current proportional to any deviation from quadrature in the LOI and LOQ signals. This current is integrated by integrator 5 to produce an error voltage. The error voltage is input to comparator 6 along with the output from VCO 1. The error voltage is used to modify the mark-space ratio of the VCO output in order to correct for inaccuracies in the VCO and divide by two circuit.
There are several problems with this approach. First, it does not correct for intrinsic errors in the phase detector and comparator (see FIG. 1B).
Second, it does not correct for any inaccuracies in the components of the quadrature modulator (mixers 31 and 32, and combiner 33, in FIG. 1A).
Third, it does not correct for inaccuracies in the quadrature of the baseband signal.
Fourth, it requires a highly accurate phase detector in order to be effective.
Fifth, since it involves making corrections to the LO signal, a high frequency signal, it is difficult to achieve satisfactory results with this approach.
Accordingly, there is a need for a system for and method of reducing or eliminating the unwanted sideband in the output of a quadrature modulator which overcomes one or more of the disadvantages of the prior art.
In accordance with the purpose of the invention as broadly described herein, there is provided a system of and method for reducing or eliminating the unwanted sideband in the output of a quadrature modulator in which the presence of the unwanted sideband is detected through an unwanted sideband detector applied to the output of the quadrature modulator. In one embodiment, the system comprises a baseband correction circuit, a quadrature modulator, and an envelope detector. The I and Q components of the baseband signal, BBI and BBQ, are input to the baseband correction circuit. The outputs of the baseband correction circuit, BBIxe2x80x2 and BBQxe2x80x2, are input to the quadrature modulator as are the I and Q components of the local oscillator signal. The output of the quadrature modulator is input to the unwanted sideband detector. The output of the unwanted sideband detector is input to the baseband correction circuit.
Any inaccuracy in the quadrature of the baseband or local oscillator signals, or in the components of the quadrature modulator, results in an unwanted sideband in the output of the quadrature modulator. The unwanted sideband detector detects the presence of an unwanted sideband detector in the output of the quadrature modulator, and provides a signal representative thereof to the baseband correction circuit. In one embodiment, the baseband correction circuit iteratively corrects the phase relationship of the I and Q components of the baseband signal responsive to the signal provided by the unwanted sideband detector until the relationship is about a quadrature relationship. In another embodiment, the process iterates one or more times until the unwanted sideband in the output of the quadrature modulator has been reduced to an acceptable level.
In one implementation, the unwanted sideband detector is an envelope detector. This implementation of the invention exploits the property that the output of the quadrature modulator can be expressed as the superposition of an AM signal, and an FM signal whose carrier frequency xcfx89c is the frequency of the wanted sideband, xcfx89BB+xcfx89LO, and whose modulation frequency xcfx89m is the frequency offset between wanted and unwanted sidebands, 2xcfx89BB. Mathemetically, this property can be expressed as follows:                               S          ⁡                      (            t            )                          =                  xe2x80x83                ⁢                                            s              fm                        ⁡                          (              t              )                                +                                    s              am                        ⁡                          (              t              )                                                              =                  xe2x80x83                ⁢                              sin            ⁡                          (                                                ω                  c                                ⁢                t                            )                                +                                    (                              B                /                2                            )                        ⁢                          (                                                sin                  ⁢                                      {                                                                  [                                                                              ω                            c                                                    +                                                      ω                            m                                                                          ]                                            ⁢                      t                                        }                                                  -                                  sin                  ⁢                                      {                                                                  [                                                                              ω                            c                                                    -                                                      ω                            m                                                                          ]                                            ⁢                      t                                        }                                                              )                                +                                                  xe2x80x83                ⁢                              sin            ⁡                          (                                                ω                  c                                ⁢                t                            )                                +                                    (                              M                /                2                            )                        ⁢                          (                                                sin                  ⁢                                      {                                                                  [                                                                              ω                            c                                                    +                                                      ω                            m                                                                          ]                                            ⁢                      t                                        }                                                  +                                  sin                  ⁢                                      {                                                                  [                                                                              ω                            c                                                    -                                                      ω                            m                                                                          ]                                            ⁢                      t                                        }                                                              )                                          
Because of this property, the amplitude of the envelope of the output of the quadrature modulator at a given instant in time is proportional to or representative of the amplitude of the unwanted sideband. Therefore, the signal output by the envelope detector is a direct measure of the unwanted sideband.
In one embodiment, a method in accordance with the subject invention comprises the steps of correcting the I and Q components, BBI and BBQ, of a baseband signal using one or more parameters, thereby forming BBIxe2x80x2 and BBQxe2x80x2; quadrature modulating BBIxe2x80x2 and BBQxe2x80x2 respectively with the I and Q components of a local oscillator, LOI and LOQ, to form an output signal; detecting an unwanted sideband from the output of the quadrature modulator; if the unwanted sideband is below a desired threshold, storing the one or more parameters; and if the unwanted sideband is not below the desired threshold, revising the one or more parameters responsive to the unwanted sideband, and returning to the correcting step. In one embodiment, the method further comprises iterating one or more times through the foregoing steps until the energy of the unwanted sideband is reduced to an acceptable level.
In one implementation, the detecting step comprises detecting the amplitude of the envelope of the output of the quadrature modulator.
An advantage of the foregoing method is that the ability to correct for any inaccuracies in the components of the quadrature modulator is provided.
Another advantage is that the ability to correct for any inaccuracies in the quadrature of the I and Q components of the baseband signal is provided.
A third advantage is that a highly accurate phase detector is not required.
A fourth advantage is that correction to a high frequency signal is not required. Instead, the invention only requires correction to the relatively low frequency baseband signal.
Additional advantages of the subject invention will be set forth in the description which follows, or will be apparent to one of skill in the art.
This application is related to co-pending U.S. patent application Ser. No. 09/514,501, filed Feb. 29, 2000, entitled xe2x80x9cSYSTEM OF AND METHOD FOR REDUCING OR ELIMINATING THE UNWANTED SIDEBAND IN THE OUTPUT OF A TRANSMITTER COMPRISING A QUADRATURE MODULATOR FOLLOWED BY A TRANSLATIONAL LOOP,xe2x80x9d and co-pending U.S. patent application Ser. No. 09/515,538, filed Feb. 29, 2000, entitled xe2x80x9cSYSTEM OF AND METHOD FOR COMPENSATING A BASEBAND SIGNAL TO REDUCE THIRD ORDER MODULATION DISTORTION,xe2x80x9d both of which are owned in common by the assignee hereof, and both of which are hereby fully incorporated by reference herein as though set forth in full.